During the past 25 to 30 years, considerable attention has been devoted to means for recovering mineral wealth from the ocean floors of the world. Particularly, recovery of petroleum from submerged oil fields has been the subject of intense research and development; moreover, recovery of manganese nodules and other ocean bottom resources has also been studied with great interest.
In the field of petroleum recovery, recent years have seen the extensive use of permanent off-shore drilling platforms, drill ships, and semi-submersible, mobile platforms. These approaches to oil prospecting and recovery have developed primarily from a desire to use the fully-developed land-based technology in the ocean environment. While drilling in submerged formations presents a whole host of problems peculiar to it alone, much, if not most, of the drilling equipment used on present-day off-shore platforms or semi-submersibles would be familiar to those in the art experienced primarily in land-based operations.
Semi-submersible platforms are in ever-increasing use today, primarily due to their inherent ability to be moved relatively easily from drilling site to drilling site, without significant disassembly. Because the elevated platform used in semi-submersibles must necessarily support large, variable masses of equipment, supplies and personnel, such devices inherently require large metacentric heights. To ensure positive stability against capsizing due to all conceivable loading and sea state conditions, semi-submersibles require the use of support columns for the drilling platform which have rather large waterplane areas where the columns pierce the water while the semi-submersible is in its submerged, operation position. For such systems, heave resonance commonly occurs in seas having periods in the 14 to 30 second range. This resonance range is adequate in most sea conditions, but, unfortunately, it is the heavier seas with wave lengths up to 305 meters and wave heights of up to 30.5 meters which are associated with periods in this range. Under the influence of such seas, present-day semi-submersibles frequently experience heave amplitudes of as much as 40% of the wave height, which may be as large as 12 to 15 meters. Whether the platform is used for oil drilling or other geological exploration, such motion usually presents serious problems and may cause all operations to cease. The economic losses due to such down time can run to millions of dollars per year for an individual semi-submersible platform.
In addition to the above problems of heave motion, semi-submersibles are rather slow, ungainly devices to move from place to place, due to their great size, low propulsion power and poor transit hydrodynamic design. Such platforms may be moved only under the most favorable sea states at low speeds. Furthermore, the extreme sea induced loading conditions experienced by the semi-submersibles require the extensive use of expensive, heavy construction steel to provide adequate safeguard against failure.
From the above, it is apparent that a need exists for a type of geological exploration and well drilling station or platform which will experience only slight heave movement during worst case sea conditions; will be capable of substantially higher speeds during transit from location to location under any sea state; and, yet, will require much less extensive use of the expensive construction materials in use in current semi-submersible platforms.